The advance of technology has driven faster operation speed and clock frequency for digital circuits such that the techniques of differential transmission lines are widely applied in high-speed data transmission applications. The differential transmission lines have the characteristics of low electromagnetic radiation, and low crosstalk, and therefore acted an important role in digital signal transmission. However, in an actual high speed circuit, the circuit for differential transmission lines will often appear some non-ideal circuit structures, for example, a circuit with asymmetrical wiring, bended signal trajectory, and the existence of slots. Those non-ideal circuit structures may adversely convert the differential-mode signals into common-mode noises, which may cause serious electromagnetic interference (EMI), and therefore interference the operation of the radio frequency circuit or the antenna.
To solve the problem of EMI, in the past, it is most common to use an electromagnetic material, for example, common-mode choke (CMC), to achieve the suppression of EMI, in which the feature of high inductance of electromagnetic material is used to suppress the generation of EMI. However, the permeability of the electromagnetic material attenuates quickly in high frequency, in which condition makes the electromagnetic material not suitably in high-speed transmission interface with frequency section of GHz or above.
Later, a common-mode reflection circuit having a defective ground structure or a mushroom structure was proposed successively. The defective ground structure or the mushroom structure constructs a resonant cavity in a symmetry plane of the common-mode reflection circuit to suppress the common-mode signal by using the resonant cavity. Here, the defective ground structure or the mushroom structure of the common-mode reflection circuit is able to effectively suppress the common-mode signal at the high frequency operation of GHz.
The above common-mode reflection circuit is a circuit for reflecting the common-mode signal. When the common-mode reflection circuit is in operation, most of the common-mode signal will be reflected to the front circuit to prevent the common-mode signal to interfere the circuit to be protected. However, the reflected common-mode signal may also be transmitted to other radiating elements, in which the problem of electromagnetic interference still exists in the communication system.